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Published August 09, 2010, 09:03 AM

NWS: Determining the strength of a tornado

Tyler, ND (WDAY TV) - The National Weather Service has rated tornado damage in the southern valley from Saturday's tornado. Near Tyler, North Dakota damage was an EF-3. Across the river near Doran, Minnesota they estimated EF4 damage with winds up to 175 miles per hour. How do they determine the tornado's strength?

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